Optical transmission schemes based on coherent detection and digital signal processing (DSP) of polarization division multiplexing (PDM) multilevel modulation formats, are attracting more attention and are being considered for next generation optical networks. The main advantage of coherent receivers is that they provide both amplitude and phase information of a received optical signal. This information is used by a DSP-based equalizer to invert the channel linear transfer function in order to recover the transmitted signal. Hence, the digital equalizer compensates for all linear channel impairments, namely chromatic dispersion (CD) and polarization mode dispersion (PMD). This is particularly true in the weakly non-linear regime of typical optical systems.
CD and PMD compensation can be obtained using a single time domain equalizer (TDE), provided that the channel memory is less than the TDE length. The inter-symbol interference (ISI) caused by PMD is typically limited, so the number of taps required in the TDE to compensate for PMD is low. In contrast, the channel memory induced by residual CD can be very long, so a high taps number is required in a TDE to compensate for CD. Another approach exploits a reduced complexity TDE, which compensates for PMD, combined with a frequency domain equalizer (FDE) which performs CD compensation. In the FDE, the signal is transformed by a Fast Fourier Transform (FFT) and multiplied by the inverse of the dispersive channel transfer function. The advantage of this solution is that the computational complexity of an FDE is lower than a TDE even for a small number of taps. However using an FDE necessitates prior knowledge of the residual CD value of the channel. In most cases, the CD is assumed to be a known quantity and is preset in the FDE. However the CD of a transmission link across an optical network can change due to dynamic switching and rerouting of the signal, which means that the uncompensated residual CD of the link is unknown. An adaptive CD compensation is therefore needed to cope with the dynamic changes of light paths in an optical network. Reported solutions to this include delay tap sampling, minimum mean-squared error (MMSE) equalizer assisted by channel estimation from a training sequence, overlap frequency domain equalizer (OFDE) with a blind estimation algorithm operating in the time or frequency domain, and using time domain least mean square (LMS) adaptive filters. These proposed solutions have various complexities, including requiring the use of a look up table, the need for a training sequence, an “Ad hoc” block to perform CD estimation, and a high number of taps for effective CD compensation.
An optical performance monitoring (OPM) algorithm, for coherent optical receivers, based on the elaboration of the TDE tap coefficients to provide fiber linear parameters information has been proposed by T. Xu et al, “Chromatic dispersion compensation in coherent transmission system using digital filters”, Optics Express, vol. 18, no. 15, pp. 16243-16257, 2010. However, in order to perform a CD estimation with good accuracy, this approach requires the use of a TDE with a high number of taps, even higher than the number of taps required by the system to reach an error free condition, further increasing the computational complexity of the TDE.